If "concerted" can also have the meaning that you describe (and that I grew up believing in), then why isn't this meaning/usage included in any definition for "concerted"?
Yes, it does. Here is the entire verb entry from that link:
1690s, "to contrive, adjust;" 1707, "to contrive and arrange mutually," from French concerter and directly from Italian concertare "to bring into agreement" (see concert (n.)). Related: Concerted; concerting
See it?
1: https://www.merriam-webster.com/dictionary/concerted
2: http://www.dictionary.com/browse/concerted
3: https://www.macmillandictionary.com/us/dictionary/american/c...
4: https://www.grammarphobia.com/blog/2011/04/concerted.html
https://en.oxforddictionaries.com/definition/concerted (entry 1.1)
https://dictionary.cambridge.org/us/dictionary/english/conce...
Note also that your link #4 references the Oxford English Dictionary, which is where the first link I give above comes from. The argument given in your link #4 seems plausible, but given the fact that there are reputable sources leaning both ways, I don't think you can say the single person usage is simply wrong. Perhaps "uncommon" might be a better term.
It's an interesting question as to what a word means when dictionaries differ. In some cases, the differences might exist on a spectrum (e.g., is "furious" more upset than "livid"). But in this case, we have a completely separate meaning that some dictionaries bless but others do not hint at.
I would be curious to see whether there are historical trends here, which might hint if we're headed in one direction or the other.
Words mean what people intend them to mean when they use them.
At least one dictionary I have lists a definition:
'done with great effort or determination: you must make a concerted effort to curb this.'
Also, when I think of myself doing a concerted effort I think of all my faculties working together, if that helps.